Wide abrasive belt type lumber planing machine

ABSTRACT

A wide abrasive belt lumber planing machine in which the planing action is performed by a contact drum-supported abrasive belt as the workpiece is moved through the machine, with its top surface in contact with the belt. An in-feed pressure shoe and an outfeed pressure shoe press down on the workpiece just as it enters and leaves the abrasion zone, and to preclude the generation of abnormally high friction loads due to variations in thickness of the workpiece, the force with which the in-feed shoe is pressed down is produced by a pressure-regulated air cylinder.

United States Patent 91 Olin WIDE ABRASIVE BELT TYPE LUMBER PLANINGMACHINE [75] Inventor: Paul H. Olin, Minneapolis, Minn. [73] Assignee:Timesavers, Inc., Minneapolis,

Minn.

[22] Filed: May 1, 1972 [2]] Appl. No.: 249,175

[52] US. Cl. Sl/l38 [51] Int. Cl B24b 21/00 [58] Field of Search 51/135R, 137-139, 51/76 [56] References Cited UNITED STATES PATENTS 554,7582/1896 Wigell et a1 51/139 X 3,208,187 9/1965 Ernst 51/76 R 3,271,9099/1966 Rutt et al.... 51/138 X 2,791,070 S/l957 Schaller ..5l/l38 Jan.1,1974

FOREIGN PATENTS OR APPLICATIONS 1,921,566 12/1970 Germany 5l/l38 PrimaryExaminer-Othell M. Simpson Attorney-lra Milton Jones [57] ABSTRACT Awide abrasive belt lumber planing machine in which the planing action isperformed by a contact drumsupported abrasive belt as the workpiece ismoved through the machine, with its top surface in contact with thebelt. An in-feed pressure shoe and an outfeed pressure shoe press downon the workpiece just as it enters and leaves the abrasion zone, and topreclude the generation of abnormally high friction'loads due tovariations in thickness of the workpiece, the force with which thein-feed shoe is pressed down is produced by a pressure-regulated aircylinder.

6 Claims, 5 Drawing Figures A112 PRESSURE 'PATENTED JAN 1 I974 SHEET 20F 3 Flea WIDE ABRASIVE BELT TYPE LUMBER PLANING MACHINE This inventionrelates to wide abrasive belt lumber planing machines, wherein anendless conveyor belt carries workpieces horizontally through themachine beneath the contact drum of an abrading headfor engagement by anabrasive belt trained around the contact drum; and the invention hasmore particular reference to improvements in such machines whereby thepower required for driving its conveyor belt can be significentlyreduced from what was heretofore required. This makes it possible to usea less expensive drive motor and drive transmission without in anywiseimpairing the operation and performance of the machine.

The abrasive head of machines of this type comprises an endless coarsegrit abrasive belt trained around two vertically spaced apart rollersthat have their axes parallel and horizontal. The lower one of theserollersis the contact drum, the axis of which is fixed with respect tothe frame of the machine during operation of the machine. The contactdrum is driven by a motor mounted on the machine frame to thus impartorbital motion to the abrasive belt normally in the direction oppositeto the movement of the workpiece through the machine. The upper rolleris an idlermounted for bodily up and down motion and biased upwardlywhen the machine is in operation to maintain tension on the abrasivebelt.

Workpieces are carried under the abrading head by the top stretch of ahorizontally oriented endless conveyor belt that is driven by a motorseparate from the one that drives the contact drum. The top'stretch ofthe endless belt is solidly supported by frame-carried structure whichincludes freely rotating rollers and a flat, horizontal platen by whichthe top working stretch of the conveyor belt is firmly supporteddirectly beneath the contact drum.

As a workpiece is carried through the machine by the conveyor belt, andjust before it comes in contact with the abrading head, and again justafter passing out from under the head, the workpiece should be firmlypressed down against the conveyor belt, so that its upper surfacetravels as nearly as possible in a horizontal plane spaced above thelowermost surface of the contact drum-supported abrasive belt a distancedetermined by the amount of stock to be planed off. Without such firmdownward pressure on the workpiece directly upstream and downstream ofits contact with the drumsupported abrasive belt, there would be noassurance of achieving accurate planing of the workpiece. It is,therefore, essential to the success of the operation to hold theworkpieces firmly down on the conveyor belt.

The means for thus pressing the workpiece down against the conveyor beltcomprises a pair of pressure shoes which extend transversely across theconveyor belt and project inwardly towards and under opposite sides ofthe contact drum to slidingly engage the workpiece in zones closelyadjacent to, but at opposite sides of, the zone of abrasion. Fartherfrom the contact drum, at each side of it, there are freely rotatablerollers that engage the upper surface of the workpeice in its passage toand from the abrasion zone. Both the pressure shoes and the freelyrotatable rollers have a degree of bodily up and down motion and arebiased downwardly so that they exert a yielding downward force on theworkpiece. Heretofore, the biasing force upon both the in-feed pressureshoe as well as the outfeed pressure shoe and all of the rollers hasbeen exerted by springs reacting against fixed parts of the machine.

The present invention is based upon a recognition that, in prior wideabrasive belt planing machines, the use of springs to impart downwardbias to the in-feed pressure shoe which engages the workpiece as itenters the abrasion zone, has been responsible for requiring the drivemotor for the conveyor belt to be considerably larger and more powerful(and, therefore, more expensive) than it really needed to be. Theheretofore unappreciated relationship between the power of the motordriving the conveyor belt and the character of the in-feed pressure shoebiasing means arises from the fact that the workpieces generally roughlumber have a substantial variation in thickness from point to pointalong their length. To understand how this thickness variation inworkpieces affects the requirements for the power of the conveyor beltmotor, consideration mut be given to the character of the biasing forcewhich a spring exerts. As a spring is flexed or loaded, the force thatit exerts is in substantially linear relation to the extent of itsdeformation. Thus, when a vertically movable pressure shoe is urgeddownwardly by a spring, the force required to lift the shoe is leastwhen the shoe is near the bottom limit of its permitted movement, andincreases as the shoe is raised.

Hence, if a workpiece of varying thickness along its length is carriedbeneath a spring biased pressure shoe on a conveyor belt, the shoeexerts the least downward pressure upon the thinnest portions of theworkpiece and the greatest downward pressure upon its thickest portions.Therefore, as the thicker portions of the workpiece pass beneath thepressure shoe, there is an increase in friction between the shoe and theworkpiece, requiring more work from the motor to keep the workpiecemoving under the shoe at a constant speed. However, the work that mustbe performed by the motor increases more than proportionally to theincreased friction between the shoe and the workpiece, owing to thecharacter of the conveyor belt. As the shoe applies increased downwardforce to the workpiece, such force is transmitted through the workpieceonto the conveyor belt which is thus forced more tightly down againstits supporting platen. As a consequence, the friction between theconveyor belt and the platen is increased and decreased simultaneouslywith, and in substantially direct proportion to, changes in frictionbetween the shoe and the workpiece.

While the art has heretofore recognized that increases in thickness ofthe workpiece as it passes under the in-feed pressure shoe raises theload on the motor driving the conveyor belt, this appreciation has takenthe form of a sort of intuitive understanding that the power rating ofthe conveyor drive motor had to be appraised on the basis of anallowance for a substantially large safety factor, for it was wellunderstood that if the motor was not oversized to a considerable extent,it could be expected to stall whenever a portion of a workpiece that wasapproaching the contact drum was substantially thicker than the nominalthickness for which the machine was set.

What seems not to have been appreciated isthat this need for oversizingthe conveyor drive motor arose from the character of the biasingforcethatwas apin frictional drag that resulted from changes in biasingforce.

With the foregoing observati rmn mind, it is the general object of thisinvention to provide a wide abrasive belt planing machine of thecharacter described which has a substantially ower horsepower conveyordrive motor than machines of equivalent capacity heretofore in use, butwhich nevertheless achieves uniformly steady motion of workpieces pastthe contact drum, despite very substantial variations in thickness ofthe workpieces along their length.

More specifically, it is an object of this invention to provide meansfor so biasing a pressure shoe in a wide abrasive belt planing machinethat frictional drag between the shoe and a workpiece engaged therebyremains substantially constant despite relatively large variations inthickness of the workpiece along its length, and whereby frictionbetween the conveyor belt that carries the workpieces and a platen thatthe belt supportingly overlies, likewise remains substantially constant.

With the above and other objects in view which will appear as thedescription proceeds, this invention resides in a novel construction,combination and arrangement of parts substantially as hereinafterdescribed and more particularly defined by the appended claims, it beingunderstood that such changes in the precise embodiment of thehereindisclosed invention may be made as come within the scope of theclaims.

The accompanying drawings illustrate one complete example of a physicalembodiment of the invention constructed according to the best mode sofar devised for the practical application of the principles thereof, andin which:

FIG. 1 is a view in side elevation ofa wide belt sanding machineembodying the principles of this invention;

FIG. 2 is a vertical sectional view through the contact drum and thepressure shoes of the machine, showing the relationships of those partsto one another and illustrating the principles of this invention more orless diagrammatically.

FIG. 3 is an enlarged view generally similar to FIG. 1, but withportions of the machine housing broken away to illustrate details ofconstruction;

FIG. 4 is an end view of the in-feed pressure shoe and its associatedstructure, said view being taken on the plane of the line 4-4 in FIG. 5;and

FIG. 5 is a view in elevation of the structure shown in FIG. 4, takenfrom the right-hand side of FIG. 4, but with portions broken away toillustrate details of construction.

Referring now more particularly to the accompanying drawings, thenumeral 5 designates generally the abrading head of a wide abrasive beltlumber planing machine, which head comprises, in general, a wide endlessabrasive belt 6 trained around a lower contact drum 7 and an uppertension roll 8. The contact drum is journalled on the frame 9 of themachine for rotation on a fixed axis and is driven in the customarymanner by a powerful motor (not shown). The tension roll 8 is mounted ina known manner for limited bodily up and down motion and is biasedupwardly to maintain tension on the abrasive belt.

workpieces are carried beneath the abrasive head, to be abraded thereby,by means of a conveyor which is designated generally by the numeral 12and which comprises a supple endless belt 13 trained around horizontallyspaced apart rollers 15 and 16 with a straight upper working stretch 17extending between those rollers. The roller 15 is driven by an electricmotor connected therewith through an appropriate transmission system, asmore or less diagrammatically illustrated in FIG. 1 at 19. In thismanner, orbital motion is imparted to the conveyor belt in the directionto move its upper stretch from right to left, as viewed in the drawings.

As is more or less conventional, the upper stretch of the conveyor beltis supported by idler rollers 20 and a flat smooth surfaced platen 21,located directly beneath the contact drum. The distance between thebottom of the contact drum-supported abrasive belt and the top surfaceof the platen thus determines the amount of stock that is removed from aworkpiece fed through the machine, and since the machine must be capableof handling a wide range of workpiece thicknesses, that distance must beadjustable. To this end, the entire conveyor is mounted on the machineframe in a known manner for bodily up and down adjustment.

As already explained, it is important that the workpiece be held in firmengagement with the platensupported conveyor belt directly adjacent tothe zone of abrading action, and especially just as it enters that zone.The necessary downward force for this purpose is applied to theworkpiece by an in-feed pressure shoe 24 and an out-feed pressure shoe25. The downward force which these pressure shoes exert on the workpiecemust be applied as close as possible to the zone of engagement betweenthe abrasive belt and the workpiece and, for this reason, the pressureshoes are shaped to have toe portions 26 that project inwardly under thecontact drum, as best seen in FIG. 2. Essentially, only these toeportions of the shoes engage the workpiece.

The pressure shoes 24 and 25 extend across the full width of theconveyor belt and are mounted on cross shafts 27 and 27 which have theirends received in coaxial bores in vertical mounting brackets 28 that arerigidly fixed to the main frame of the machine, only one of thesebrackets being shown in the drawings. The pressure shoes are preferablycastings and have flat parallel end walls 29. Between these end wallsthe cross sectional shape of the shoes is essentially uniform anddefined by a concave front surface 30 and a convex back surface 31 whichis stepped, as at 32, to define the heel of the shoe. Forwardly of thisheel the bottom surface 33 of the shoe slopes downwardly to join thebotto n st rface34 of the toe of the shoe.

The end walls 29 of the shoes have coaxial bores to receive the crossshafts 27 and 27 whereby each shoe is rockably supported by itsrespective cross shaft. For the out-feed shoe 25 which requires nobodily vertical adjustment, the cross shaft 27 and the bore in the endwalls 29 of this shoe are coaxial; but in the case of the in-feed shoe24 which must be bodily vertically adjustable, the connection betweeneach of its end walls 29 and the cross shaft 27 includes an eccentric 35keyed to the shaft and journalled in the end wall of the shoe. Theeccentrics at the opposite ends of the shoe are, of course, coaxial andbeing keyed to the cross shaft it follows that by rotation of the shaftthe axis about which the shoe rocks can be raised and lowered betweenlimits determined by the throw of the eccentrics. The need for suchvertical adjustment of the axis about which the shoe 24 rocks arisesfrom the variations in the level of the lowermost surface of theabrasive belt occasioned by differences in grit size. Rotation can beimparted to the shaft 27 to effect this adjustment by means of a wrenchapplied to the head of a cap screw 36 that is threaded into one end ofthe shaft, or, if desired, a handle could be fixed to the shaft.

Since the shaft 27 on which the out-feed shoe is rockably mounted doesnot have to turn, it is fixed with respect to the mounting brackets 28by which it is supported. The cross shaft 27, however, must turn bothwith respect to the mounting brackets and the shoe. Axial displacementof this shaft is precluded by collars 37 (only one being shown) clampedto the shaft just inside the end walls 29 of the shoe. This secures theshaft against shifting axially with respect to the shoe, and theconfinement of the shoe between the two mounting brackets (one at eachside of the machine) completes the securement of the shaft againstendwise movement.

Another collar 38 on the extreme end of the shaft into which the capscrew 36 is threaded provides for securing the shaft in a selectedposition of rotary adjustment. For this purpose, the collar is keyed tothe supporting bracket 28, as at 39, and is releasably clamped to theshaft.

Since the out-feed pressure shoe 25 engages the workpiece surface afterit has been abraded, so that the elevation of that surface is fairlyuniform, it is entirely satisfactory to use a compression spring 40 oneat each end of the shoe to impart force to the shoe in the direction topress its toe portion against the workpiece.

As explained hereinbefore, the inevitable variations in thecross-sectional dimensions of nominally the same size rough lumberresults in corresponding variations in the elevation of the top surfaceof the workpieces moving through the machine. To accommodate thosevarying conditions, the in-feed pressure shoe 24 must be capable of upand down motion. While the prior art use of springs to force the in-feedshoe down onto the workpiece did not preclude such up and down motion,it often resulted in the exceptionally severe friction loads referred tohereinbefore, which sometimes were so great that they stalled theconveyor drive motor. To prevent this from happening, a higherhorsepower motor was used. This, of course, entailed a significantincrease in the cost of the machine, not only because the motor was moreexpensive, but also because the transmission system had to be capable ofhandling the increased loads.

This invention eliminates these objectionable consequences of blindlyfollowing the deeply entrenched prior practice of using spring force tohold both pressure shoes down, by employing regulated fluid pressure tohold the in feed shoe down onto the workpiece. Specifically, an airpressure cylinder 41 is mounted at each end of the in-feed shoe by beingsecured to a bracket 42 that is welded to the inner face of the adjacentframe'carried mounting bracket 28. These air cylinders are preferably ofthe conventional diaphragm type, having a plunger 43 connected to andprojecting from one side of a diaphragm 44 to be forced outwardly of thecylinder by air pressure manifested at the opposite side of thediaphragm.

The outer end of the plunger 43 bears against an upwardly projecting arm45 on the pressure shoe 24, and to accommodate arcuate motion of thisarm, the plunger applies its thrust to the arm through a swivelconnection 46.

Uniformity in the pressure applied to the workpiece by the in-feed shoe24, regardless of variations in thickness of the workpiece, is assuredby the incorporation of a pressure regulator valve 47 in the airpressure line leading to the cylinder. As will be understood, this valvemaintains the pressure in the cylinder at the selected value throughoutthe full range of diaphragm motion that accompanies the rocking movementof the pressure shoe. Hence, no matter what the distance might bebetween the underside of the toe of the shoe and the platen-supportedconveyor belt, the load on the drive motor remains the same.

An adjustable stop 48 limits downward rocking movement of the toeportion of the pressure shoe. This stop consists simply of the head of abolt that passes freely through a hole in the bracket 42 and is threadedinto the upper portion of the shoe. Lock nuts on the bolt secure itagainst loss of adjustment.

As is customary in machines of this type, there are spring urgedhold-down idler rolls 50 that maintain the workpieces in firm engagementwith the conveyor belt in the conventional way, but a feature of theinvention is the utilization of the structure that rockably mounts thetwo pressure shoes 24 and 25 to also mount two of the idler rollsnamely, those nearest the shoes. Thus, each of these rolls is freelyrotatably mounted on a cross shaft 52, the ends of which are secured toand connect a pair of arms 53 that are located beyond the ends of therolls. These arms are pivotally mounted on the cross shafts thatrockably mount the pressure shoes and project beyond the back sides ofthe shoes. At the free ends of the arms 53 there are extensions 54 intowhich bolts 55 are threaded to project upwardly therefrom and have theirheaded ends pass through holes in horizontal ledges 56 that are weldedto the mounting brackets 28 and to the adjacent brackets 42.

Compression springs 57 encircling the bolts 55 are confined between theunderside of the ledges 56 and nuts 58 threaded onto the bolts, toyieldingly maintain the idler rolls at a predetermined level tangent tothe plane containing the underside of the toe portions of the pressureshoes when no workpiece is present.

Although the drawings show only the structure at one side of themachine, it will no doubt be understood that the illustrated structureis essentially duplicated at the other side of the machine.

Those skilled in the art will appreciate that the invention can beembodied in forms other than as herein disclosed for purposes ofillustration.

The invention is defined by the following claims:

1. A wide abrasive belt lumber planing machine having a frame, a contactdrum rotatably mounted in the frame, a conveyor belt that has ahorizontally extending Working stretch which is solidly supported byframe carried means including a platen beneath the contact drum, and bywhich workpieces are carried under the drum to be abraded by anorbitally moving abrasive belt trained around it, and power drive meansfor the conveyor belt, said machine also having a pressure shoe near thecontact drum, a portion of which engages workpieces to confine themtightly against the conveyor belt directly before they are engaged bythe abrasive belt, said machine being characterized by:

A. means mounting the pressure shoe on the machine frame for rockingmovement about a horizontal axis transverse to the direction workpiecesare carried through the machine by said conveyor belt, whereby theworkpiece engaging portion of the shoe can move arcuately up and down;

B. pneumatic force-applying means fixedly mounted on the machine frameat a level above the pressure shoe and having a plunger which is movablesubstantially horizontally;

C. a rigid arm projecting upwardly from the pressure shoe andconstrained to rock therewith, said arm having a pad which is engaged bysaid plunger whereby the plunger transmits to the pressure shoe theforce which is applied by the pneumatic force-applying means; and

D. means including a pressure regulating valve for supplying compressedair to said pneumatic forceapplying means in such manner that the forceproduced thereby remains substantially constant.

2. A wide abrasive belt lumber planing machine having a frame, a contactdrum rotatably mounted in the frame, a conveyor belt that has ahorizontally extending working stretch which is solidly supported byframe carried means including a platen beneath the contact drum, and bywhich workpieces are carried under the drum to be abraded by anorbitally moving abrasive belt trained around it, and power drive meansfor the conveyor belt,

said machine also having a pressure shoe near the contact drum, aportion of which engages workpieces to confine them tightly against theconveyor belt directly before they are engaged by the abrasive belt,

said machine being characterized by: A. means mounting the pressure shoeon the machine frame for rocking movement about a horizontal axistransverse to the direction workpieces are carried through the machineby the conveyor belt, with the work engaging portion thereof spaced fromsaid horizontal axis and located closely adjacent to the lowermostportion of the contact drum to move arcuately up and down and hold aworkpiece down on the conveyor belt; and B. pneumatic biasing meansacting on the pressure shoe in a direction to urge its workpieceengaging portion downward towards the conveyor belt with a yieldingforce that remains constant irrespective of the distance between saidportion of the shoe and the conveyor belt, said pneumatic biasing meanscomprising v 7' V I. an air cylinder mounted on a fixed part of themachine at an elevation above that of said horizontal axis with the axisof the cylinder substantially horizontal, the air cylinder having aplunger which is projected axially from the cylinder by air pressuretherein,

2. an arm fixed with respect to the pressure shoe and projectingupwardly from said horizontal axis about which the pressure shoe rocks,

said plunger bearing against said arm whereby pressure in the pneumaticcylinder forces the work engaging portion of the pressure shoe downward;and

C. a pressure regulating valve through which air under pressure issupplied to the air cylinder.

3. The machine of claim 2, wherein said means that rockably mounts thepressure shoe on the machine frame includes a shaft extending across themachine with its end portions journalled in bearings in the machineframe, and on which the pressure shoe is rockably supported,

and further characterized by:

A. a hold-down roll to hold workpieces down on the conveyor belt as theyapproach the pressure shoe;

B. a pair of arms rockably mounted on said shaft and projectingsubstantially horizontally therefrom and upstream of the work advancingtravel of the conveyor belt,

said pair of arms having the hold-down roll journalled therein; and

C. biasing means reacting between each of said pair of arms and themachine frame to yieldingly force the hold-down roll downward.

7' 4. The machine of claim 1, further characterized by: lost motionconnection means connected between said rigid arm and a part fixed withrespect to the frame of the machine for defining a limit of downwardmotion of the workpiece engaging portion of the pressure shoe.

5. The machine of claim 1, further characterized by:

said means for rockably mounting the pressure shoe including aneccentric, rotation of which provides for up and down adjustment of theaxis about which the pressure shoe rocks.

6. The machine of claim 5, wherein:

A. said means for rockably mounting the pressure shoe comprises a crossshaft supported at its opposite ends from fixed parts of the machine,and having the shoe rotatably mounted thereon with its workpieceengaging portion spaced horizontally in one direction from the crossshaft; and further characterized by:

B. a pair of roll carrying arms also mounted on said cross shaft forswinging motion about the shaft and for bodily up and down adjustingmotion, said roll carrying arms extending horizontaly from the shaft inthe direction away from the workpiece engaging portion of the shoe;

C. a roll freely rotatably mounted on the free end portion of said armsfor bodily up and down movement with swinging motion of said arms;

D. spring means reacting between the roll carrying arms and the machineframe to bias the roll carrying arms downwardly; and

E. motion limiting means connected between the machine frame and thesaid arms for maintaining said roll at not less than a predetermineddistance above the platen.

1. A wide abrasive belt lumber planing machine having a frame, a contactdrum rotatably mounted in the frame, a conveyor belt that has ahorizontally extending working stretch which is solidly supported byframe carried means including a platen beneath the contact drum, and bywhich workpieces are carried under the drum to be abraded by anorbitally moving abrasive belt trained around it, and power drive meansfor the conveyor belt, said machine also having a pressure shoe near thecontact drum, a portion of which engages workpieces to confine themtightly against the conveyor belt directly before they are engaged bythe abrasive belt, said machine being characterized by: A. meansmounting the pressure shoe on the machine frame for rocking movementabout a horizontal axis transverse to the direction workpieces arecarried through the machine by said conveyor belt, whereby the workpieceengaging portion of the shoe can move arcuately up and down; B.pneumatic force-applying means fixedly mounted on the machine frame at alevel above the pressure shoe and having a plunger which is movablesubstantially horizontally; C. a rigid arm projecting upwardly from thepressure shoe and constrained to rock therewith, said arm having a padwhich is engaged by said plunger whereby the plunger transmits to thepressure shoe the force which is applied by the pneumatic force-applyingmeans; and D. means including a pressure regulating valve for supplyingcompressed air to said pneumatic force-applying means in such mannerthat the force produced thereby remains substantially constant.
 2. Awide abrasive belt lumber planing machine having a frame, a contact drumrotatably mounted in the frame, a conveyor belt that has a horizontallyextending working stretch which is solidly supported by frame carriedmeans including a platen beneath the contact drum, and by whichworkpieces are carried under the drum to be abraded by an orbitallymoving abrasive belt trained around it, and power drive means for theconveyor belt, said machine also having a pressure shoe near the contactdrum, a portion of which engages workpieces to confine them tightlyagainst the conveyor belt directly before they are engaged by theabrasive belt, said machine being characterized by: A. means mountingthe pressure shoe on the machine frame for rocking movement about ahorizontal axis transverse to the direction workpieces are carriedthrough the machine by the conveyor belt, with the work engaging portionthereof spaced from said horizontal axis and located closely adjacent tothe lowermost portion of the contact drum to move arcuately up and downand hold a workpiece down on the conveyor belt; and B. pneumatic biasingmeans acting on the pressure shoe in a direCtion to urge its workpieceengaging portion downward towards the conveyor belt with a yieldingforce that remains constant irrespective of the distance between saidportion of the shoe and the conveyor belt, said pneumatic biasing meanscomprising
 2. an arm fixed with respect to the pressure shoe andprojecting upwardly from said horizontal axis about which the pressureshoe rocks, said plunger bearing against said arm whereby pressure inthe pneumatic cylinder forces the work engaging portion of the pressureshoe downward; and C. a pressure regulating valve through which airunder pressure is supplied to the air cylinder.
 3. The machine of claim2, wherein said means that rockably mounts the pressure shoe on themachine frame includes a shaft extending across the machine with its endportions journalled in bearings in the machine frame, and on which thepressure shoe is rockably supported, and further characterized by: A. ahold-down roll to hold workpieces down on the conveyor belt as theyapproach the pressure shoe; B. a pair of arms rockably mounted on saidshaft and projecting substantially horizontally therefrom and upstreamof the work advancing travel of the conveyor belt, said pair of armshaving the hold-down roll journalled therein; and C. biasing meansreacting between each of said pair of arms and the machine frame toyieldingly force the hold-down roll downward.
 4. The machine of claim 1,further characterized by: lost motion connection means connected betweensaid rigid arm and a part fixed with respect to the frame of the machinefor defining a limit of downward motion of the workpiece engagingportion of the pressure shoe.
 5. The machine of claim 1, furthercharacterized by: said means for rockably mounting the pressure shoeincluding an eccentric, rotation of which provides for up and downadjustment of the axis about which the pressure shoe rocks.
 6. Themachine of claim 5, wherein: A. said means for rockably mounting thepressure shoe comprises a cross shaft supported at its opposite endsfrom fixed parts of the machine, and having the shoe rotatably mountedthereon with its workpiece engaging portion spaced horizontally in onedirection from the cross shaft; and further characterized by: B. a pairof roll carrying arms also mounted on said cross shaft for swingingmotion about the shaft and for bodily up and down adjusting motion, saidroll carrying arms extending horizontaly from the shaft in the directionaway from the workpiece engaging portion of the shoe; C. a roll freelyrotatably mounted on the free end portion of said arms for bodily up anddown movement with swinging motion of said arms; D. spring meansreacting between the roll carrying arms and the machine frame to biasthe roll carrying arms downwardly; and E. motion limiting meansconnected between the machine frame and the said arms for maintainingsaid roll at not less than a predetermined distance above the platen.